(1) Field of the Invention
The present invention relates to a method of forming a crystal, particularly to a crystal forming method for growing a single-crystal by subjecting a substrate having a free surface on which a non-nucleation surface and a nucleation surface are arranged adjacent to each other to a crystal forming process. The nucleation surface has a greater nucleation density than the non-nucleation surface with respect to a material with which the single-crystal will be formed. The nucleation surface has an area sufficiently small to selectively permit only one nucleus to form, which will grow to form a single-crystal. The method of the present invention forms a crystal which is used, for example, as an electronic, optical, magnetic, piezoelectric or surface acoustic element of a semiconductor integrated circuit, an optical integrated circuit, or a magnetic circuit.
(2) Related Background Art
This application is an improvement in the process for forming a single crystal on an insulating substrate as described in copending application Ser. No. 07/409,284, filed Sep. 19, 1989, the disclosure of which is incorporated herein by reference.
In the prior art, single crystal thin films to be used for semiconductor electronic devices or optical devices have been formed by epitaxial growth on a single crystal substrate. For example, it is known that epitaxial growth of Si, Ge, GaAs, etc., can be performed from the liquid phase, gas phase or solid phase on Si single crystal substrate (silicon wafer), and it is also known that epitaxial growth of a single crystal such as GaAs, GaAlAs, etc., occurs on a GaAs single crystal substrate. By use of the semiconductor thin film thus formed, semiconductor devices and integrated circuits, electroluminescent devices such as semiconductor lasers or LED have been prepared.
Also, much research and development has been recently conducted concerning ultra-high speed transistors by use of two-dimensional electronic gas, ultra-lattice devices utilizing quantum well, etc. What has made such research possible is the high precision epitaxial technique such as MBE (molecular beam epitaxy) or MOCVD (organometallic chemical vapor deposition) by use of ultra-high vacuum.
In such epitaxial growth on a single crystal substrate, it is necessary to take into account matching of lattice constants and coefficient of thermal expansion between the single crystal material of the substrate and the epitaxial growth layer. For example, although it is possible to effect epitaxial growth of Si single crystal film on sapphire which is an insulating single crystal substrate, the crystal lattice defect at the interface due to deviation in lattice constant and diffusion of aluminum which is a component of sapphire to the epitaxial layer pose problems in application to electronic devices or circuits.
Thus, the method for forming a single crystal thin film of the prior art by epitaxial growth may be understood to De dependent greatly on its substrate material. Mathews et have examined combinations of the substrate material with epitaxial growth layer (EPITAXIAL GROWTH, Academic Press, New York, 1975, ed. by J. W. Mathews).
The growth of a crystalline silicon on a crystalline silicon substrate is essentially two-dimensional growth, which is formed layer-by-layer by atomic arrangement on the lattice of the single crystal substrate.
Accordingly, during epitaxial growth on a single crystal, the presence of matching lattice constants and matching coefficients of thermal expansion between the single crystal material and the epitaxial growth layer make possible the formation of a uniform single crystal. The order in the single crystal substrate is carried over and matched during epitaxial growth.
To the contrary, when a material is deposited on a chemically dissimilar substrate, there is a lack of matching lattice constants and a lack of matching coefficients of thermal expansion. That lack causes a disordered growth. Accordingly, when a layer is grown over an amorphous substrate, the structure in the overlayers will also be amorphous, or polycrystalline at best. The absence of long-range order in the amorphous substrate is reflected in the absence of long-range order in the overlayer. In general, when a material is grown on a chemically dissimilar substrate, the growth sequence generally observed is nucleation, coalescence of the nuclei and then recrystallization. That nucleation-coalescence-recrystallization sequence in any subsequent grain growth will produce an overlayer without long range order. Generally, such overlayers are amorphous overlayers, or, at best, polycrystalline overlayers, which are aggregates of small random-sized crystal grains with grain boundaries between adjacent grains.
Both silicon oxides and silicon nitrides are stable amorphous materials, which have been widely used for silicon integrated circuit processing. Such amorphous compounds are formed by chemical vapor deposition. When silicon was deposited on amorphous silicon oxide, or amorphous silicon nitride, an amorphous layer or at best, a polycrystalline silicon layer, was formed with a variety of grain sizes. That is because the silicon layer was formed by spontaneous nucleation, coalescence and recrystallization. In that disordered system, the crystal grain boundary locations were randomly determined, since the silicon nuclei were randomly formed over the substrate surface. Closely located nuclei formed small grains, while more remote nuclei formed larger grains due to the difference in time before interaction with adjacent grains.
The formation of that heterogeneous structure having random polycrystalline grains was an obstacle to applying amorphous materials for production of crystal electronic devices. The nonuniform grain boundaries acted as electrical potential barriers and degraded the characteristics of the device.
Also, the size of the substrate is presently about 6 inches for a Si wafer, and the enlargement of GaAs and sapphire substrate is further inhibited. In addition, since the single crystal substrate is high in production cost, the cost per chip becomes higher.
Thus, for production of a single crystal layer capable of use in a device of good quality according to the method of prior art, it is a problem that the kinds of the substrate materials are limited to an extremely narrow scope.
On the other hand, research and development of three-dimensional integrated circuits to accomplish high integration and multi-function by laminating semiconductor devices in the normal line direction of the substrate have been made often in recent years. Also, research and development of large area semiconductor devices such as solar batteries or switching transistors of liquid crystal picture elements, etc, in which devices are arranged in an array on a cheap glass, are becoming more common from year to year.
What is common to both of these is that the technique for forming a semiconductor thin film on an amorphous insulating material and forming an electronic device such as transistor, etc. thereon is required. Among them, particularly the technique for forming a single crystal semiconductor of high quality on an amorphous insulating material has been desired.
Generally speaking, when a thin film is deposited on an amorphous insulating material substrate such as SiO.sub.2, etc., due to the defect of long distance order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline. Here, the amorphous film refers to a state in which near distance order to the extent of the closest atoms is preserved, but no longer distance order exists, while the polycrystalline film refers to single crystal grains having no specific crystal direction gathered as separated at the grain boundaries.
For example, in the case of forming Si on SiO.sub.2, according to the CVD method, if the deposition temperature is abut 600.degree. C. or lower, it becomes an amorphous silicon, while it becomes a polycrystalline silicon with grain sizes distributed between some hundred to some thousand .ANG. at a temperature higher than said temperature. However, the grain sizes and their distribution of polycrystalline silicon will be varied greatly depending on the formation method.
Further, by melting and solidifying an amorphous or polycrystalline film by an energy beam such as laser or rod-shaped heater, etc., a polycrystalline thin film with great grain sizes of some microns or millimeters has been obtained (Single crystal silicon on non-single-crystal insulator, Journal of Crystal Growth, vol., 63, No. 3, Oct. 1983, edited by G. W. Gullen).
When a transistor is formed on the thus formed thin film of respective crystal structures and electron mobility is measured from its characteristics, mobility of about 0.1 cm.sup.2 /V .cndot. sec or less is obtained for amorphous silicon, mobility of 1 to 10 cm.sup.2 /V .cndot. sec for polycrystalline silicon. having grain sizes of some hundred .ANG., and a mobility to the same extent as in the case of single crystalline silicon for polycrystalline silicon with great grain sizes by melting and solidification.
From these results, it can be understood that there is great difference in electrical properties between the device formed in the single crystal region within the crystal grains and the device formed as bridging across the grain boundary. In other words, the deposited film on the amorphous material obtained in the prior art becomes amorphous or polycrystalline structure having grain size distribution, and the device prepared thereon is greatly inferior in its performance as compared with the device prepared on the single crystal layer. For this reason, the uses are limited to simple switching devices, solar batteries, photoelectric converting devices, etc.
On the other hand, the method for forming a polycrystalline thin film with great grain sizes by melting and solidification had problems. A great deal of time is required to form the film due to scanning of amorphous or single crystal thin film with energy beam for every wafer. Therefore, that technique is poor in bulk productivity, and also, it is not suited for area enlargement.
Further, in recent years, studies of diamond thin film growth are becoming popular. Diamond thin film, which is particularly broad in bandgap as 5.5 eV as the semiconductor, can be actuated at higher temperature (about 500.degree. C. or less) as compared with Si, Ge, GaAs, etc., which are semiconductor materials of the prior art. Also, the carrier mobility of both electrons and positive holes surpass that of Si (1800 cm.sup.2 /V .cndot. sec from electrons, 1600 cm.sup.2 /V .cndot. sec for positive holes), and thermal conductivity is also extremely high. For this reason, it has been expected to be promising for application in semiconductor devices of the great consumption power type with great heat generation quantity.
However, although there have been reports in the prior art about epitaxial growth of diamond thin film on a diamond substrate by vapor phase growth (N. Fujimoto, T. Imai and A. Doi Pro. of Int. Couf. IPAT), there is no successful report about heteroepitaxial growth on a substrate other than diamond substrate.
Generally speaking, diamond nuclei are generated by utilizing excitation with microwaves, using a hydrocarbon type gas such as CH.sub.4, etc., and by irradiation with a hot filament or an electron beam, but the nucleation density is generally low, whereby a continuous thin film can only De obtained with difficulty. Even if a continuous thin film may be formed, it has a polycrystalline structure with great grain size distribution and is difficult to apply for semiconductor devices.
Also, as long as a diamond substrate is used, it is expensive as a matter of course, posing also a problem in enlargement of area. Thus, it is not suitable for practical application.
As described above, in the crystal growth method of the prior art and for the crystal formed thereby, three-dimensional integration or enlargement of area could not be done with ease, was difficultly applied in practice for devices, and crystals such as single crystals and polycrystals, etc., required for preparation of devices having excellent characteristics could not be formed easily and at low cost.
In the field of silicon on insulator (SOI) technology for growing a single-crystal on an insulating substrate, a method of forming crystals has been reported in, for example, "Extended Abstracts of the 19th SSDM 191" by T. Yonehara et al., 1987 (hereinafter referred to as "the first report"). In this method, nuclei are selectively generated by utilizing the difference in nucleation density between materials forming the surface of the substrate. Crystals are grown around the nuclei.
This crystal forming method will be described with reference to FIGS. 1 to 3.
First, as shown in FIG. 1, a substrate 401 having a surface 403 with a small nucleation density is prepared, and regions 407 and 407' with a diameter a and with a nucleation density greater than that of the surface 403 are arranged on the substrate 401 at a pitch of b.
Next, as shown in FIG. 2, the substrate 401 is subjected to a predetermined crystal forming process, whereby nuclei 409 and 409' made of a deposited substance (the substance with which the crystals are to be formed) are generated only on the surfaces of the regions 407 and 407', respectively, whereas no nuclei are generated on the surface 403. Throughout the specification, surfaces corresponding to the surfaces of the regions 407 and 407' will be referred to as "nucleation surfaces", and those corresponding to the surface 403 will be referred to as "non-nucleation surfaces".
Subsequently, as shown in FIG. 3, the nuclei 409 and 409' generated on the nucleation surfaces of the regions 407 and 407', respectively, are grown so that they grow beyond the nucleation surface regions 407 and 407' until, finally, the crystal grain 410 grown from the nucleation surface 407 abuts on the crystal grain 410' grown from the adjacent nucleation surface 407' to define a grain boundary 411.
Hitherto, the above-described crystal forming method has been performed in more than one manner. The first report described an example in which amorphous Si.sub.3 N.sub.4 was the material used to form a plurality of nucleation surfaces arranged at desired positions on the substrate, while SiO.sub.2 was the material used to form the non-nucleation surfaces. In this example, a Si single-crystal was formed on each nucleation surface by a chemical vapor deposition (CVD) method. Another example is reported in a second report ("The 35th Lecture Meeting on Applied Physics", 28p-M-9, 1988), in which SiO.sub.2 was the material used to form non-nucleation surfaces, and a plurality of regions to provide nucleation surfaces were formed by using a focused ion beam to implant Si ions into desired positions of the non-nucleation surfaces. In this example, a plurality of Si single-crystals were formed by a CVD method.
However, the crystal forming method as described in the first report entails the following problems concerning control over the crystal formation. This is because under certain crystal growth conditions, the difference in nucleation density between Si.sub.3 N.sub.4 and SiO.sub.2 is insufficient, corresponding to a ratio of about 1000:1 at most.
If an insufficient difference in nucleation density between the nucleation surfaces and the non-nucleation surfaces is to be remedied by controlling the crystal forming conditions to thereby achieve a sufficient nucleation density on the nucleation surfaces, this may result in unnecessary nucleation on the non-nucleation surfaces. Conversely, if an insufficient difference is to be remedied by adopting a low nucleation density level to thereby suppress nucleation on the non-nucleation surfaces, this can result in an unnecessarily low nucleation density on the nucleation surfaces. If such is the case, some of the nucleation surfaces, where single-crystals should grow, may not have any nuclei generated thereon to grow into single-crystals. Hence, these surfaces may not be able to grow single-crystals.
If substances, such as Si.sub.3 N.sub.4 and SiO.sub.2, having compositions determined by stoichiometric ratios are used, the nucleation densities are determined based on a one-to-one relationship under the given crystal growth conditions. For this reason, it is sometimes difficult to determine, under certain crystal growth conditions adapted to avoid unnecessary nucleation on the non-nucleation surfaces, a particular nucleation density on the nucleation surfaces which assures that a crystal grown from a single nucleus is formed on each of the nucleation surfaces. It is also difficult to suppress, under certain crystal growth conditions adapted to assure that a nucleus is generated and grows into a single-crystal, unnecessary generation of nuclei and unnecessary growth of crystals on the non-nucleation surfaces. Thus, under certain crystal growth conditions, it is difficult to fill a large area with good-quality crystal grains, in which grain size and the grain boundary positions are successfully controlled. As a result, it is difficult to improve the yield of the crystals.
It was an intention of the crystal forming method described in the second report to eliminate the problems of the first-report method. The second method used nucleation surfaces consisting of regions where the nucleation density is increased by ion implantation employing a focused ion beam.
However, under the described circumstances, forming a plurality of nucleation surfaces by projecting a focused ion beam takes a long time. When the substrate has a large surface area, a long processing time is required, leading to a decrease in productivity which renders the second method not readily applicable. In order to overcome this drawback, a method was proposed in Japanese Patent Laid-Open No. 107016/1988, in which nucleation surfaces are formed in the following manner: a mask of a photoresist is patterned by a resist process so that it has openings only at positions corresponding to the regions where ions should be implanted; then, ion implantation is performed throughout the surface of a substrate over the mask, thereby implanting ions only in those portions of the substrate surface which are to serve as the nucleation surfaces. With this method, however, when ion implantation is performed at a high dose, the photoresist may have its properties changed in the vicinity of the openings, thereby making stripping of the photoresist difficult. Nuclei tend to be generated on any remaining photoresist and incomplete removal of the photoresist can diminish the level of control over the selective growth of crystals. As a result, a plurality of poly-crystalline grains having uncontrolled crystal grain size and uncontrolled grain boundary positions may be formed. For this reason, improvement in the yield has been sought.
In brief, none of the above efforts to overcome the problems of the method as described in the first report have proven to be sufficiently successful. They involve problems such as requiring long processing time, high cost, decrease of controllability and low yield.
The present invention solves the problem of forming crystals over non-monocrystalline substrate surfaces. In order to provide a self-matching single crystal film on a non-monocrystalline substrate, two growth factors must be controlled. The crystal growth must grow on a nucleation surface of (1) sufficiently small area and (2) of sufficient nucleation density to selectively grow only a single nucleus. The single nucleus grows to form a crystal. By patterning the nucleation surface, large grains of single crystals can be formed having well-defined and uniform grain boundaries.